From A Wiki of Ice and Fire
A mercenary is a soldier who takes temporary assignments fighting for wages rather than as an official member of an army. There are a number of different types of mercenaries. The terms "sellsword" and "freerider" carry a certain stigma in Westeros. Sellswords are said to have no loyalty, and freeriders no discipline.
A Sellsword is a mercenary who hires out his services to the highest bidder. Inevitably, this sort of lifestyle involves a great deal of violence and physical exertion. Many sellswords are organized into companies. Some of the sellsword companies are very disciplined (such as the Golden Company), and some are nothing but rabble joined together in search of loot (like the Brave Companions); the Second Sons and the Stormcrows are in the middle. Most tend to be experienced professional soldiers, as it is a profession a man tends to chose after he's tasted a few battles and learned that he's good at fighting.
The Free Cities have made heavy use of mercenaries for centuries, to fight their endless wars in the Stepstones and the Disputed Lands; as such there are more sellswords in Essos than in Westeros. Many of the mercenary soldiers of the east are organized into long-established sellsword companies, or "free companies".
Notable Sellsword companies
"Freerider" is a broad term, sometimes used to denote a mounted sellsword, but more often referring to other mounted fighters who are not part of a lord's retinue or feudal levy. Some are hedge knights and are experienced fighters, but also many are green and untrained recruits and farm boys who have nowhere else to go. Most do not collect wages, instead fighting for plunder or a hope to be taken into a lord or knight's service. Some freeriders fight simply as a means to survive: when war sweeps over villages, those refugees who do not flee to the nearest city or take to outlawry as "broken men" may saddle their horse (if they're lucky enough to have one) and become a freerider for a better chance of being fed. Whatever their origin, freeriders are mostly used as scouts, outriders, foragers, and light cavalry. 
A sellsail is a mercenary sailor who engages in naval battles for pay. Sellsails may work as pirates or smugglers when not employed.
|“||A man who fights for coin is loyal only to his purse.||”|
|“||Loyal sellswords are rare as virgin whores.||”|
|“||Never trust a sellsword.||”|
|“||There are old sellswords and bold sellswords but there are no old, bold sellswords.||”|
|“||An old sellsword is a cautious sellsword.||”|
|“||Sellswords have no pity.||”|
|“||Whatever their sires or their grandsires might have been back in Westeros before their exile, the men of the Golden Company were sellswords now, and no sellsword could be trusted.||”|
|“||Wealthy knights from Houses old in honor did not cross the narrow sea to sell their swords, unless exiled for some infamy.||”|
References and Notes
- ↑ So Spake Martin: Military Questions, June 21, 2001
- ↑ 2.0 2.1 2.2 So Spake Martin: Mercenaries, May 13, 2000
- ↑ A Game of Thrones, Chapter 69 , Tyrion, p 765.
- ↑ A Clash of Kings, Chapter 57, Sansa, p 598.
- ↑ A Dance with Dragons, Chapter 50, Daenerys.
- ↑ A Storm of Swords, Chapter 57, Daenerys.
- ↑ A Feast for Crows, Chapter 33, Jaime.